1. Field of the Invention
The present invention relates to a semiconductor device provided with a circuit including a semiconductor element such as a transistor, and a method for manufacturing the semiconductor device. For example, the present invention relates to a power device which is mounted on a power supply circuit; a semiconductor integrated circuit including a memory, a thyristor, a converter, an image sensor, or the like; an electro-optical device typified by a liquid crystal display panel; a light-emitting display device including a light-emitting element; and an electronic device on which any of the above is mounted as a component.
In this specification, a semiconductor device means all types of devices which can function by utilizing semiconductor characteristics, and an electro-optical device, a light-emitting display device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
Transistors formed over a glass substrate or the like are manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Although transistors manufactured using amorphous silicon have low field-effect mobility, they can be formed over a larger glass substrate. On the other hand, although transistors manufactured using polycrystalline silicon have high field-effect mobility, they are not suitable for being formed over a larger glass substrate.
In contrast to transistors manufactured using silicon, attention has been drawn to a technique in which a transistor is manufactured using an oxide semiconductor, and such a transistor is applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique in which a transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based metal oxide as an oxide semiconductor, and such a transistor is used as a switching element of a pixel or the like of a display device.
Patent Document 3 discloses a technique in which in a staggered transistor including an oxide semiconductor, a highly conductive oxide semiconductor containing nitrogen is provided as buffer layers between a source region and a source electrode and between a drain region and a drain electrode, and thereby the contact resistance between the oxide semiconductor and the source electrode and between the oxide semiconductor and the drain electrode is reduced.
Non-Patent Document 1 discloses a top-gate oxide semiconductor transistor in which a channel region, a source region, and a drain region are formed in a self-aligned manner.
In the above-described manner, transistors including an oxide semiconductor have been developed. In an oxide semiconductor, however, oxygen deficiency serves as a donor and causes release of an electron which is a carrier. It is difficult to completely remove oxygen deficiency in an oxide semiconductor, resulting in difficulty in controlling the threshold voltage of a transistor including an oxide semiconductor.